Method to produce composite material with a hard inner layer with deep draw capability

ABSTRACT

A method for producing a composite metallic material having a soft outer layer of a first metal or metal alloy and a hard inner layer of a second metal or metal alloy includes first selecting the soft layer and the hard layer according to the desired properties of the combined layers. The soft layer is then bonded with the hard layer. Finally, the bonded layers are annealed at a temperature within the range of 700-1200 degrees Fahrenheit to secure the bond and enhance formability.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority from Provisional PatentApplication No. 62/009,773, filed on Jun. 9, 2014, the disclosure ofwhich is relied upon and incorporated herein by reference.

FIELD OF THE INVENTION

A method to produce a highly formable metallic composite material withhard inner layers (e.g., stainless metal) and a soft outer layer (e.g.,copper) is developed incorporating a unique method for bonding andannealing the layers.

BACKGROUND OF THE INVENTION

Traditionally, hard metallic layers (e.g., stainless steel)bonded/adhered to soft layers (e.g., copper) could not be fully annealedfor certain applications because fully annealing the stainless steelwill result in unacceptable surface finish or properties. For example,in the cookware industry, stainless steel is used as the inner layer ofthe vessel to ensure a nonreactive surface during cooking and tofacilitate cleaning. However, the stainless steel lacks good heattransfer characteristics. The desired heat transfer characteristics canbe provided by an aluminum or copper outer layer (e.g., C11000) becauseof the excellent heat conductivity of these materials. In this case,fully annealing the stainless steel inner layer will cause large grainsin the soft aluminum or copper outer layer. The large copper grains inreturn, cause an unacceptable surface finish (e.g., orange peeling)after forming operations.

A common method to circumvent the grain growth issue is by using alloyedcopper (such as C19400) which pin grain boundaries and reduce graingrowth. But alloyed copper is not only considerably more expensive andnot widely used, but also results in a much lower thermal/electricalconductivity and performance in most applications. FIG. 1 provides acomparison of different copper properties and the associated price.

SUMMARY OF THE INVENTION

A method for producing a composite metallic material having a soft outerlayer and a hard inner layer is described herein. The method includesthe steps of first identifying the soft layer of a metal or metal alloyand the hard layer of a metal or metal alloy according to the desiredproperties of the combined layers. The soft layer is then bonded withthe hard layer. Finally, the bonded layers are annealed at a temperaturewithin the range of 700-1200 degrees Fahrenheit to secure the bond andenhance formability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a table providing comparison of different copper propertiesand the associated prices.

FIG. 2 is a flow chart illustrating the process for joining materials asdescribed herein.

FIG. 3 is a diagram illustrating the bonding mill for cladding a softmetallic layer with a hard metallic layer.

FIG. 4 is a table identifying a non-exclusive list of hard metalliclayers that can be combined with soft metallic layers according to theprocess described herein.

DETAILED DESCRIPTION OF THE INVENTION

A cold-roll-bonding and annealing process is described herein to producea clad metallic material composite 10 that includes a soft outer layer12 of a metal or metallic alloy and a hard inner layer 14 of a metal ormetallic alloy that will allow the use of any type of soft outer layer(e.g., C1100 copper) and which is formable in deep draw or comparableoperations. As a bit of background, deep drawing is a sheet metalforming process in which a sheet metal blank is radially drawn into aforming die by the mechanical action of a punch. It is thus a shapetransformation process with material retention. The process isconsidered “deep” drawing when the depth of the drawn part exceeds itsdiameter.

The clad composite 10 includes two roll bonded metal layers with thesoft outer layer 12 (such as aluminum, copper or related alloymaterials) and the hard inner layer 14 (such as stainless steel, steel,titanium or related alloy materials). A multitude of clad combinationsare available to combine the unique surface properties of the variousmetals and metal alloys to suit the particular application or designneeds, such as providing the desired light weight, heat transfer, and/orstrength characteristics. In this disclosure, a hard layer is consideredto be any metal with a Rockwell hardness on the B scale of greater thanHRB 50 and a soft layer is any metal with a Rockwell hardness on the Bscale of less than HRB 50.

The cold roll bonding process is used to produce a bi-layer of hard/softcomposite material 10. The bond between the metallic layers 12, 14 canthen be secured with a heat treating/sinter process. The sinter processdoes not fully anneal or recrystallize the hard inner layer but securesthe bond between the layers 12, 14. As long as the hard layer 14 is usedas the inner layer in any forming process, a sinter anneal is adequateas the stainless steel inner layer only experiences compressive forces.Referring to the same example above, cookware products have stainlesssteel on the inside (cooking surface) and copper on the outside. In sucha case, the stainless steel will only see compressive forces and fullanneal heat treatment is not required. Pure copper is sufficient in thiscase resulting in a significant increase in the thermal conductivity.This will also result in lower production costs and reducedmanufacturing cycle time.

The steps involved in this process 100 are shown in flowchart of FIG. 2.Specifically, the user will select two layers 12, 14; namely, the userwill select a clean soft layer (step 102) and a clean hard layer (step104) according to the desired properties for the resultant materialcomposite 10. Once the two layers 12, 14 are chosen, the next step 106is for the layers 12, 14 to be bonded using a conventional cold-rollbonding process. The bonding process 106 that is employed is similar toknown cold-roll bonding process, in which the two layers 12, 14 arebond/clad on a four-high mill 20, as shown in FIG. 3. Such a process isdescribed in prior patents, such as U.S. Pat. Nos. 6,475,675 and8,420,225, which are incorporated herein by reference.

The bonded layers 10 are then sintered/annealed in step 108, but notaccording to conventional annealing practices. That is, the bondedlayers 10 are sintered/partially-annealed at about 700° F.-1200° F. toimprove the bond/peel strength between the individual layers 12, 14. Inthe known processes, the bonded layers 10 are traditionally manufacturedby annealing the copper and the stainless steel at high temperatures(1800° F.), which causes the grain growth in the copper and hence theneed for an alloyed copper.

The annealing process 108 is performed in a batch or continuous processand in a controlled atmosphere. Annealing can be performed inatmospheres such as hydrogen, nitrogen, or a mixture ofhydrogen/nitrogen. The temperature of the sinter/anneal is determined bytwo factors: sensitization temperature and copper grain growth.

Concerning sensitization temperature, austenitic stainless steel (ifused in this type of product) usually cannot be batch annealed as itwill go through a ‘sensitization’ process where the chromium is depletedfrom the grain boundaries due to chromium carbide precipitationresulting in poor stress corrosion cracking performance.

With respect to copper grain growth, the temperature needs to be lowerthan the grain growth temperature of the copper (softer) material 12.

It should also be noted that ferritic stainless steel undergoes anembrittlement when subjected to prolonged heating between 750 degreesFahrenheit and 1000 degrees Fahrenheit. The most severe effects areexperienced around 885 degrees Fahrenheit. Such embrittlement causesdecrease in the ductility/forming characteristics.

Therefore, the selected temperature is generally in the range of700-1200° F. depending on the material selection. The range of greaterthan 700 degrees Fahrenheit and less than 1200 degrees Fahrenheit allowsfor batch annealing of Austenitic stainless steel if used in the productand use of copper alloys that have higher oxygen content in hydrogenatmospheres. Batch annealing is a traditionally less expensive processthan continuous or strand annealing. Copper alloys with a high oxygencontent when annealed at high temperatures (such as 1800° F.) in ahydrogen atmosphere form water vapor within the copper causing blistersin the copper surface.

After the bonded material 10 has been annealed, it is transitioned topost process in step 110. The post process 110 usually varies with thedifferent applications, and may include further rolling, slitting, cutto length and forming operations that would follow thesintering/annealing process. For example, in the cookware industry, theannealed material 10 will be cut to sheets, formed into pots ofdifferent sizes and shapes, buffed to produce a cooking vessel. Incomparison, in the electronics industry, the bonded material 10 could bestamped to different shapes and sizes and used as a heat sink or otherpurposes for the electronic component.

By using this process, various combinations of metal layers are possiblethat were not considered previously possible using prior techniques.Moreover, it is noted that the process can be used on a variety of hardand soft layers to achieve the desired properties. A few additionalexamples (although a variety of other examples may be clad according tothe described process) are found in the table included as FIG. 4. Forexample, titanium and copper may be combined to provide a materialcomposite that lightweight and corrosion resistant, which is good forthe cookware and electronics industries. Steel and copper may becombined to provide a material composite that is especially good for thecookware and cable shield industries, in that the composite has improvedthermal and electrical conductivity, improved strength, and can beproduced at a lower cost. A third example is the combination ofstainless steel with copper, which provides improved thermalconductivity and corrosion resistant, which is also beneficial in thecookware industry.

Having thus described exemplary embodiments of a method to producemetallic composite material, it should be noted by those skilled in theart that the within disclosures are exemplary only and that variousother alternatives, adaptations, and modifications may be made withinthe scope of this disclosure. Accordingly, the invention is not limitedto the specific embodiments as illustrated herein, but is only limitedby the following claims.

What is claimed is:
 1. A method for producing a composite metallicmaterial having a soft outer layer and a hard inner layer comprising thesteps of: a. identifying the soft layer and the hard layer according tothe desired properties of the combined layers; b. bonding the soft layerwith the hard layer; and c. annealing the bonded layers at a temperaturewithin the range of 700-1200 degrees Fahrenheit to secure the bond andenhance formability.
 2. The method as described in claim 1 wherein stepa) further comprises selecting the soft outer layer from a groupconsisting of copper or aluminum.
 3. The method as described in claim 1wherein step a) further comprises the step of selecting the soft outerlayer having a Rockwell hardness on the B scale of less than HRB
 50. 4.The method as described in claim 1 wherein step a) further comprises thestep of selecting the hard outer layer having a Rockwell hardness on theB scale of greater than HRB
 50. 5. The method as described in claim 1wherein step a) further comprises selecting the hard outer layer from agroup consisting of steel, stainless steel, or titanium.
 6. The methodas described in claim 1 wherein step b) includes cold-roll bonding thesoft layer with the hard layer.
 7. The method as described in claim 1wherein the annealing performed in step c) is in a continuous processand in a controlled atmosphere.
 8. The method as described in claim 7wherein the annealing is performed a controlled atmosphere selected fromthe group consisting of hydrogen, nitrogen, or a mixture of hydrogen andnitrogen.
 9. A method for producing a composite metallic material havinga soft outer layer and a hard inner layer comprising the steps of: a.identifying the soft layer and the hard layer according to the desiredproperties of the combined layers; b. feeding the soft layer and thehard layer into a bonding mill to bond the soft layer with the hardlayer; and c. annealing the bonded layers at a temperature of greaterthan 700 degrees Fahrenheit and less than 1200 degrees Fahrenheit tosecure the bond and enhance formability.
 10. The method as described inclaim 9 wherein step a) further comprises selecting the soft outer layerfrom a group consisting of copper or aluminum.
 11. The method asdescribed in claim 9 wherein step a) further comprises the step ofselecting the soft outer layer having a Rockwell hardness on the B scaleof less than HRB
 50. 12. The method as described in claim 1 wherein stepa) further comprises the step of selecting the hard outer layer having aRockwell hardness on the B scale of greater than HRB
 50. 13. The methodas described in claim 12 wherein step a) further comprises selecting thehard outer layer from a group consisting of steel, stainless steel, ortitanium.
 14. The method as described in claim 9 wherein step b)includes cold-roll bonding the soft layer with the hard layer.
 15. Themethod as described in claim 9 wherein the annealing performed in stepc) is in a continuous process and in a controlled atmosphere.
 16. Themethod as described in claim 15 wherein the annealing is performed acontrolled atmosphere selected from the group consisting of hydrogen,nitrogen, or a mixture of hydrogen and nitrogen.